The invention relates to a method for the production of composite bodies, of cylindrical ceramic composite bodies for sheathed-element glow plugs in particular, each of the piece parts to be joined being preformed from different materials by cold pressing, joined in a joining step, bonded in a hot pressing operation and subsequently brought to their final shape in a fabricating operation, as well as a press which is configured as a multi-cavity mold and is used for the production of the ceramic composite bodies
In such a known method implemented by Robert Bosch GmbH for the production of cylindrical ceramic bodies for sheathed-element glow plugs, a ceramic insulating layer is first produced by a cold pressing method and the conducting layers located to both sides of the insulating layer are produced separately. The insulating layer is subsequently inserted between the conducting layers and then initially joined in a cold pressing step. The resin is then cross-linked by a hot pressing method and the composite is formed. Finally, the composite plates are sawn into square bars and are profile ground for their fabrication. Due to the two press steps required for producing a ceramic composite body, namely the cold pressing and the hot pressing, dissimilar press molds are required, it being also necessary to switch from one press mold to another. Moreover, the division of the hardened ceramic composite plates into square bars and their subsequent grinding to a round shape is time-consuming and expensive.
The object of the present invention is to provide an economical production method for cylindrical ceramic composite bodies from previously cold pressed piece parts which are suitable for the production of ceramic sheathed-element glow plugs, as well as a press configured as a multi-cavity mold which can be used with this method and can sharply lower the production costs. In this connection, several particular properties of the resin for binding the ceramic powder must be taken into account; thus, for example, the hot pressed composites should not be removed from the press mold until after it has cooled since the hot cylindrical ceramic composite bodies are deformed or even damaged under the influence of demolding forces.
In an exemplary embodiment of the present invention, the joining step is made up of one step in which the piece parts are inserted into a shaping sleeve and each of the two ends of the shaping sleeve is closed by an axially movable ram. Next, at least one shaping sleeve prepared in this manner is placed into a corresponding bore hole of an insulating plate and from there is pressed into a bore hole of a hot supporting plate of steel which is aligned with this bore hole, the supporting plate being in parallel contact with the insulating plate for a short time. Next, the heated supporting plate with the shaping sleeve is clamped between two press plates of a press and then both ends of the piece parts located in the shaping sleeve are pressed with spring-loaded press pins which axially engage the end rams of the clamping sleeve and pass through the two press plates which are also heated. Next, the pressed piece parts remain under pressure in the shaping sleeve and the latter remains in the hot supporting plate between the hot press plates for a necessary pressure keeping period. Next, the press pins are then retracted, the press is opened, the supporting plate is withdrawn from the press plates and the compound pressed in the shaping sleeve is further hardened in the hot supporting plate without pressure. Finally, the shaping sleeve is ejected from the hot supporting plate and cooled, and the cooled cylindrical molding is subsequently removed from the shaping sleeve, the time-consuming grinding of square bars to a round shape is thereby avoided. Since the press used can be set up as a 100-cavity mold, the production of such cylindrical ceramic composite bodies for ceramic sheathed-element glow plugs is considerably simplified and the costs are considerably reduced by the increase of throughput thus attained.
Such a press designed as a multi-cavity mold which is suitable for the production method in an exemplary embodiment of the invention has a supporting plate, whose thickness preferably corresponds to the length of the shaping sleeves, is designed with parallel and regularly spaced bore holes which are drilled perpendicularly through the supporting plate with a diameter adapted to contain the shaping sleeves and with second bore holes lying in the center plane of the supporting plate for the circulation of a heating medium or to contain cartridge heaters. This supporting plate is clamped between two press plates using centering elements so that individually spring-loaded press pins movably arranged in corresponding bore holes of the press plate are aligned with the bore holes in the supporting plate and the guide sleeves of the individually spring-loaded press pins are each connected with a movable pressure plate on the side of the press plates facing away from the supporting plate.